There is no "evidence," but one must ask, why else would a man would pose as a police officer to bring a gun into a theme park? Do you have any ideas? I don't. Sounds awfully suspicious to me.
So there certainly is a possibility that he intended to shoot people. I'd say it's likely based on the situation, but that's up for interpretation, I guess. Anyway, security thankfully prevented us from ever knowing if that was his intention or not.
I happen to believe it was worth preventing that possibility.
Post hoc ergo propter hoc. This is a fallacy. It's hardly ever true. You have no evidence to substantiate your "belief" that the coincidental choice to have him go through the metal detector prevented a shooting inside the park. I have a theory that the more vocal open/concealed carry proponents simply want to be able to bully or scare people by carrying their firearms and use safety as a red herring and I suspect he is one of those people, carrying a weapon to show how "tough" he is and that no one can "mess" with him. The article is a little vague, but he admitted right away that he had a firearm in his pocket. People intent on committing crimes would most likely try to flee or put up a fight.
But someone could shoot others anywhere. At a hotel, in a parking lot, at a golf course, at Disney Springs. But the MK is a bigger target. Crowded places are also bigger targets. Lessening the heavy crowds right outside of the Magic Kingdom would likely discourage a psychopath from shooting or bombing, because his act or terror would be less effective.
Which is why most sane, intelligent people see these procedures as nothing more than "theater". It might slightly help prevent a mild disagreement from escalating into a "rage" incident, but it will not stop a terrorist (which is why these procedures were implemented). As has been mentioned before in this thread and by myself and others in other threads, the security checkpoints can just as easily be a target, and possibly be more effective as far as casualties are concerned, more so than a target inside the parks.